Concussion has been known by many names, including mild traumatic brain injury (TBI) and commotio cerebri. In 2001, the Concussion in Sports Group created a broad definition of concussion at the 1st International Symposium on Concussion in Sport.¹ This definition has been revised in subsequent meetings, culminating in the current definition of concussion from the 4th International Conference in 2012:²

An important aspect of this definition is the recognition of concussion as primarily a functional, rather than a structural injury. While the pathophysiology of concussion is incompletely understood, an underlying “acute metabolic cascade” has been suggested.³ According to the current hypothesis, mechanical forces from rotational injury cause disruptive stretching of axons and neuronal cell membranes. A variety of changes have been described, including cell depolarization, ionic shifts, release of neurotransmitters, alteration of glucose metabolism, and changes in blood flow patterns. Although concussion grading scales were used extensively in the past, their reliance on LOC and amnesia at the time of injury had little prognostic value. Therefore, grading scales have been abandoned, and replaced by more individualized assessment and management.¹,⁴

Over the last decade, there has been an increase in the percentage of high school students participating in team sports,⁵ with a concurrent increase in reported concussions sustained by both high school and collegiate athletes.⁶ From 2001 to 2009, emergency department (ED) visits for sports and recreation-related (including falls due to skateboarding, bicycling, etc.) TBIs sustained by those aged 19 and younger rose by 62%.⁷ However, there was no significant rise in hospitalizations from these ED visits. The increase in ED visits is likely related to both true incidence of concussion as well as heightened awareness leading to increased reporting and diagnosis.⁷,⁸ The Centers for Disease Control and Prevention (CDC) estimates that as many as 3.8 million sport-related TBIs occur each year in the US.⁹ Concussion represents between 9% and 15% of all high school sport-related injuries and over 90% of injuries to the head and face.¹⁰,¹¹ However, studies show that as many as 47% of high school athletes fail to report concussions.¹² In collegiate sports, concussion has been reported as 5.8% of all injuries,¹⁰ with one study of NCAA Division I football players showing 23.4% of players indicating that they did not intend to report potential concussion symptoms.¹³

Sports and recreational activities are a common cause of head injury throughout childhood, adolescence, and into young adulthood.⁷

While males suffer a greater overall number of sports- and recreation-related TBIs,⁶ some studies suggest that females may be at higher risk for diagnosis of concussion in gender-comparable sports, such as soccer and basketball.⁶,¹¹,¹⁴ It has also been reported that female athletes have a larger number and greater severity of symptoms after injury, in addition to an increased risk of prolonged recovery.¹⁵ A variety of hypotheses have been proposed to explain these findings, such as increased vulnerability for females due to smaller head size, weaker neck muscles, or limited dynamic head and neck stabilization compared to male athletes.¹⁶ Increased reporting among female athletes has also been proposed as an explanation; however, this would not account for the greater deficits seen on neuropsychological testing observed in female athletes after concussion.¹⁵

As concussion is primarily a functional injury, the diagnosis remains clinical. The variability of concussion symptoms makes concussion a difficult and sometimes controversial diagnosis (Table 20.1).